I design wind tunnels among other things. That’s my job. It’s a fun job and I get to travel all over working on wind tunnels and all sorts of facilities. Because of this, I have a deep appreciation for historic wind tunnels. Like the one above.
The wind tunnel pictured above was designed and built by the Wright Brothers. It has a maximum wind speed of about 35 mph, which was plenty for them. It allowed them to test models of their Wright Flyer before sticking Wilbur in the real thing and putting his butt on the line.
Since then, wind tunnels have grown, become more specialized, and have become much more technologically advanced. There are climatic wind tunnels, low speed aerodynamic wind tunnels, transonic wind tunnels, supersonic and hypersonic wind tunnels. They are used for testing everything from automobiles to aircraft to spacecraft to skiers.
A wind tunnel is a fairly simple device. In a closed-loop tunnel, a fan blows air around a circuit. A contraction just before the test section speeds up the wind using a venturi effect. Usually there is a large settling chamber just ahead of this contraction where the wind speed is near zero. Using Bernoulli’s equation, the wind speed is calculated using the settling chamber static pressure (which is the total pressure since air speed if minimal) and the static pressure in the nozzle. Transonic, supersonic and hypersonic wind tunnels will used compressed air or even compressed, supercooled nitrogen to simulate or replicate the Mach numbers required.
Aerodynamic wind tunnels are some of the simplest, but also the most precise. The aerodynamics on a vehicle, aircraft or spacecraft can be affected by the test section walls. Therefore, the test sections are usually very large to move the walls as far away from the vehicle as possible. Other systems like slotted walls can negate this effect by creating a suction along the walls that tricks the air into behaving as if a wall is not there. Many newer aerodynamic tunnels are also being used for acoustic research and have acoustic treatments in them to deaden the amount of sound transmitted by the facility so that only the sound transmitted from the vehicle is measured. More recently, rolling road systems are being used to simulate the aerodynamic effects of a the road under the vehicle.
Climatic wind tunnels, on the other hand, can be temperature controlled, humidity controlled, have full solar simulation system (using lights and filters to simulate the spectrum and heat of the sun), and even snow and rain simulation. For these tunnels, huge refrigeration, steam and other systems are used to get the capabilities required.
The purpose of wind tunnels is to collect data. The data acquisition systems can be simple, off-the-shelf type systems or more complex, high-speed systems. One of the most impressive is the system at the National Full-Scale Aerodynamic Complex (NFAC) at NASA’s Ames Research Center (hell, that entire facility is impressive). It can measure almost 1,000 channels at up to 80 MB/sec. It is one of the fastest data acquisition systems in existence.
The nice thing about wind tunnels is they are based on physics. Physics doesn’t change. There are still ways to innovate, but the same concept that the Wright Brothers used on their wind tunnel in 1903 is the same concept that we are using on the design of wind tunnels today.